1. Technical Field
Embodiments consistent with the present invention are related to a circuit for driving a line, and, in particular, a voltage-mode line driving circuit having adaptive impedance matching.
2. Discussion of Related Art
In today's communication industry, coaxial and twisted-pair cables are widely used in transmitting E1/T1 data due to their practicability and low cost. However, as more media, particularly streaming video and audio, are transmitted over E1/T1 lines, traditional transceiver products are encountering increasing technical difficulties in transmitting and receiving the greater bandwidths of data over greater distances. For example, a line driver which is typically used to improve the transmission reliability, especially over long distances, may encounter problems with signal reflections as the signal is transmitted. To minimize reflections, the driver's output impedance should be equal to, or matched with, the characteristic impedance of the line, and be independent of process, temperature, and load variations. Moreover, the output waveform shape must meet ITU template standards.
Traditionally, there have been two ways to implement impedance matching for line drivers. FIG. 1A shows a line driver having impedance matching according to a first design. As shown in FIG. 1A, matching resistors 10 and 20 are serially connected to a voltage amplifier output 30, used to drive a voltage over line 40 which is coupled to load 50, through a transformer. However, the source voltage and power of a circuit using impedance matching as shown in FIG. 1A is consumed by matching resistor 10 or 20. Due to the voltage and power drop, a higher output voltage is required at the source end to provide a signal at the far end of line 40 with an acceptable voltage amplitude. The higher output voltage requirements make such a circuit difficult to incorporate into modern circuits which strive for smaller size, lower voltage, and lower power.
FIG. 1B shows a line driver having impedance matching according to a second design. As shown in FIG. 1B, a matching resistor 60 is connected in parallel to a current amplifier output 70. The circuit shown in FIG. 1B uses a current output to eliminate the problem that signal voltage amplitude is restricted by the source voltage, as illustrated in FIG. 1A. Although the circuit shown in FIG. 1B may be able to use a lower voltage supply (for example, 3.3 V), the current and power is similarly consumed by matching resistor 60.
With advancements in sub-micrometer processing and fabrication techniques, a C-mode line driver has been developed and widely used. It uses a current-mode signal as a driving signal and also has internal impedance matching. The C-mode line driver successfully overcomes some of the problems, such as, for example, large power consumption and power supply voltage restriction, that occur in the traditional line drivers shown in FIGS. 1A and 1B.
FIG. 2A illustrates a schematic of a conventional C-mode line driver circuit 200. As shown in FIG. 2A, the input voltage Vin is converted to a current output Io via a transconductance amplifier 202, which drives current output Io through resistor 206 creating an output voltage Vout. Output voltage Vout is sampled and then fed back to the input side of another transconductance amplifier 204, which outputs a feedback current Ifb accordingly. By sampling output voltage Vout to output feedback current Ifb which is fed back to transconductance amplifier 204, the input impedance is matched to the output impedance.
As shown in FIG. 2A, it is expected that an output voltage increment, denoted v will also cause a current variation, denoted i. This output current variation i is caused by, and depends on, the internal resistance R204 of transconductance amplifier 204. According to Ohm's Law,v=i×R204.Moreover, the output impedance Zout of circuit 200 is given by:Zout=v÷i=R204.Thus, the active output impedance is R204.
FIG. 2B is a schematic showing the circuit implementation of the C-mode line driver circuit 200 shown in FIG. 2A. As shown in FIG. 2B, transconductance amplifier 202 is achieved in the circuit through resistor 208 and 1:N current mirror 210. Moreover, the internal resistance of transconductance amplifier 204 is equivalent to the inverse of the resistance of resistor 212. Based on these relationships, the following show that the output impedance Zout of circuit 200 depends on the resistance of resistor 208.
                    R        202            =                                                  N              +              1                                      R              208                                ⁢                                          ⁢                      R            204                          =                  1                      R            212                                ;              i      =                        v                      R            208                          +                              v                          R              208                                ×          N                      ;              Z      out        =                  v        i            =                                    R            208                                1            +            N                          .            
Because resistor 208 is an on-chip resistor, the resistance R208 will vary with varying process conditions and varying temperatures. Accordingly a tuning circuit may be utilized for the adjustment of Zout to ensure that Zout matches an input impedance of load 206.
Conventional C-mode line drivers that successfully implement internal impedance matching eliminate the signal voltage loss or signal current loss occurring to the traditional driver shown by the circuits in FIGS. 1A and 1B, which can be as much as a −6 dB loss. Moreover, C-mode line drivers, produced using sub-micrometer processes, are able to be easily integrated as on-chip solutions.
C-mode line drivers, however, are unable to be implemented via a fully differential circuit. This is because they can only be constructed to have a dual-end output by duplicating two single-end output drivers, which leads to higher circuit complexity and higher static power consumption. Moreover, C-mode line drivers have poor common-mode suppression due to lack of a common-mode feedback circuit, which affects the transmitted signal quality. In addition, output impedance Zout has to be preset, and cannot be auto-tuned.
There is, therefore, a need for a driver which drives the line with a voltage-mode signal and features a simple structure with high power efficiency, better common mode rejection and the capability of adaptive impedance matching.